The present invention relates to an image forming apparatus for forming an image on a sheet and, more particularly, to a binding device for binding a stack of sheets each carrying an image formed by the image forming apparatus.
A modern image forming apparatus, e.g., a copier, laser printer or facsimile transceiver is operable with a finisher capable of sorting or stacking sheets each carrying an image thereon and binding a stack of such sheets. A current trend is toward a miniature finisher with versatile functions from the value added standpoint. Regarding the binding function, various kinds of systems have been proposed in, among others, the printing art and may generally be classified as follows.
(I) Pasting
Pasting is a traditional scheme and usually pastes the cut edges of the back of printings. To prevent the pasted printings from being loosened, a piece of cloth may be additionally pasted to the back of the printings. A binding device using this kind of scheme is disclosed in, for example, Japanese Patent Laid-Open Publication No. 253992/1990.
(II) Fastening at Center
This binding system is orthodox and quite popular with, for example, magazines. The system is such that sheets stacked in order of page are provided with a stapling portion at the center thereof and then folded in two at the stapling portion.
(III) Stapling after Folding
This binding system folds a stack of printings in, for example, two and then staples the folded stack. For example, Japanese Patent Laid-Open Publication No. 196677/1987 proposes a binding device having a single sheet staple tray and binding means in the form of a stapler provided on the tray. The binding device folds, for example, sheets of A3 size such that opposite side edges thereof do not overlap each other, staples the resulting sheet stack of A4 size in the stable tray, and releases an end fence supporting one edge of the sheets to cause the sheet stack to fall from the staple tray into another tray disposed below the staple tray by gravity.
(IV) Stapling Sheet Stack with Front and/or Rear Cover
After a front and/or a rear cover has been laid on the top or the bottom of a sheet stack, the former is stapled together with the latter. This binding scheme is becoming popular in the finishers art.
(V) Simply Stapling Sheet Stack
A sheet stack is directly bound by a stapler. This is the most simple system and may implement a binding device for a copier in the manner taught in Japanese Patent Laid-Open Publication No. 33065/1990 by way of example. The binding device has a single sheet staple tray and a stapler provided on the tray. After a sheet stack has been stapled in the staple tray, an end fence supporting one edge of the sheets is shifted in the intended direction of sheet transport, i.e., sideways to discharge the stapled stack from the tray.
The problem with all the conventional binding systems (I)-(V) described above is that they cannot achieve high binding quality, increase the area for the installation of the binding device as well as the cost, and require troublesome handling.
Specifically, the system (I) involves a number of steps including cutting, pasting, and pasting a piece of cloth and, therefore, cannot be mechanized or automated without resorting to a large scale, complicated and, therefore, expensive device. The system (II) is simple, but it has to staple a sheet stack fully arranged in order of page and, therefore, requires a complicated procedure for page arrangement. Moreover, the stapled stack is apt to become loose at the center thereof. The system (III) needs a complicated step in folding printings in two and exposes the bent portions of staples to the outside. Such bent portions not only degrade the appearance of stapled stacks but also often catch the operator's fingers. Another problem with the system (III) is that the device is bulky since major part of the stapler body is located above and outside of the sheet area in order to staple a folded stack. The binding device disclosed in Japanese Patent Laid-Open Publication 196677/1987 causes a stapled stack to fall from the staple tray to the underlying tray by gravity. This is undesirable since the individual sheets of the stack are apt to fold during the fall and to become loose due to the air pressure and physical friction acting on the stack, the impact at the end of the fall, etc. The systems (IV) and (V) also have a problem that the bent portions of staples are bare and, therefore, apt to catch the operator's fingers while degrading the appearance. This is especially true when the sheet stack is relatively thin, since the bent portions of staplers would noticeably protrude from the stack.
On the other hand, in the binding devices taught in Japanese Patent Laid-Open Publication Nos. 196677/1987 and 33065/1990, a hammering section and a seat included in the stapler are rotatably supported in the vicinity of and outside of the sheet stacking region. Hence, such a device can bind a sheet stack only at the ordinary left binding margin of the latter (corresponding to image information written in horizontal lines). It follows that the stapling position will be reversed in the right-and-left direction when it comes to sheets on which image information written in vertical lines are reproduced by an image forming apparatus. Locating a stapler at both sides of a sheet stack to allow a sheet stack to be bound at the right margin thereof would increase the overall size and cost of the binding device.